1. Field of the Invention
The present invention relates to a switched reluctance motor.
2. Description of the Prior Art
A conventional switched reluctance motor is disclosed in, for example, Switched Reluctance Drive for Electric and Hybrid Vehicles (1996 Society of Automotive Engineers, Inc.). As shown in FIGS. 7 and 8, this switched reluctance motor includes a housing 100, a stator 101 fixed in an inner bore of the housing 100 and formed by laminating electromagnetic steel plates and a rotor 102 disposed in the stator 101 and formed by laminating electromagnetic steel plates. The rotor 102 is fixed to an output shaft 103 which is rotatably supported on brackets 104 and 105 that are fixed to the housing 100 and is thereby rotatably disposed in the stator 101. The rotor 102 has three pairs of rotor pole portions 102a which project outward in the diametrical direction and which extend in the axial direction. The stator 101 has four pairs of opposing stator pole portions 101a which project inward in the diametrical direction and which extend in the axial direction. Each of the stator pole portions 101a is opposed to each of the rotor pole portions 102a in response to the rotation of the rotor 102 and a certain clearance is maintained between the stator pole portions 101a and the rotor pole portions 102a which are opposed to each other. On each of the stator pole portions 101a, a coil 107 is wound thereon. The coils 107 which are wound on each of the pairs of opposing stator pole portions 101a are connected in series with each other and thereby a magnetic flux is generated between each pairs of stator pole portions 101a when current is supplied to the coil 107. A magnetic attracting force results between the rotor pole portions 102a and the stator pole portions 101a which are approaching each other. This magnetic attracting force is changed by controlling the supply current by means of switching elements in response to the rotational position of the rotor 102, whereby motoring torque is produced.
The current supplied to the coil, when the two pairs of stator pole portions 101a are approached by two pairs of rotor pole portions 102a, is switched on and off in such a manner as to generate a pulse. In general, the current is switched on when a pair of rotor pole portions begins to be aligned with a pair of stator pole portions, and the current is switched off before a pair of rotor pole portions is aligned with a pair of stator pole portions. As a result, the magnetic attracting force increases proportionally while the current is supplied, and disappears in a moment when the current is switched off. On one hand, motoring torque is obtained by this magnetic attracting force. On the other hand, the stator pole portions are attracted radially to the rotor pole portions by this magnetic attracting force, respectively, whereby the stator and the housing are strained. When the magnetic attracting force disappears, the strain of the stator returns suddenly and simultaneously the housing is suddenly pressed outward in the diametrical direction by the stator. This impulsive variation of the housing generates periodically in response to the rotation of the rotor, whereby vibration of the housing causes objectionable acoustic noise.
In the above prior switched reluctance motor, this noise increases most at the portion A in FIG. 8. The reason for that is the stiffness against the above magnetic attracting force is high at both ends of the housing which is fixed to the brackets and is low at the central portion of the housing in the axial direction. Therefore, the amplitude of the vibration increases at the central portion of the housing in the axial direction. Accordingly, if the stiffness of the portion of the housing in which the amplitude of the vibration is large is increased, it is possible to reduce the noise. In the above prior switched reluctance motor, in order to take this measure, it is necessary to increase the back-yoke thickness W.sub.SC of the stator and the housing thickness W.sub.H at the A portion in FIG. 8. However, since the housing is normally made of aluminum, even if the housing thickness W.sub.H is increased a little, it is impossible to expect the increase of the stiffness of the housing. Further, when the back-yoke thickness W.sub.SC of the stator is increased, it is not easy to increase the back-yoke thickness W.sub.SC of only the central portion from the point of view of manufacturing. Accordingly, an increase in the weight of the motor is unavoidable.